1. Technical Field
The present invention relates to a fiber optic collimator system that disposes optical fibers with GRIN lenses being opposingly arranged and holding arbitrary optical elements, a fiber optic collimator array comprising a plurality of optical fibers with GRIN lenses being positioned in parallel, a method of manufacturing the fiber optic collimator system, and a method of manufacturing the fiber optic collimator array system that disposes the fiber optic collimator arrays in an opposed arrangement.
2. Description of the Related Art
In conventional optical information transmission, an outgoing light which is a light emerged from one optical fiber, is converted to a parallel light by a collimator lens. The parallel light after propagation is collected by another collimator lens, and is input to another optical fiber. Such an optical system is called a “collimator system” whereby a diverse optical module can be constructed by inserting various optical elements such as a filter, an optical isolator element, an optical switch, an optical modulator, or the like, between the collimator lenses. A convex lens is typically used as the collimator lens. However, a cylindrical graded index lens (hereinafter referred to as “GRIN lens”) is used to facilitate the installation. As shown in FIG. 1, when a refractive index n of the GRIN lens, as viewed from its cross-sectional direction x, y, is defined in the following equation (1), the GRIN lens shows the maximum refractive index at a cylindrical core axis. The refractive index decreases continuously in a second degree curve as being directed away from the core toward the peripheral direction. A lens effect is carried out according to this refractive index distribution.n=n0{1−g2r2/2}  (1)
wherein reference character g denotes a constant showing a light-collecting ability of the GRIN lens, n0 denotes a refractive index of a material of the GRIN lens, and r denotes a distance (r2=x2+y2) in a radial direction.
As shown in FIG. 1, provided that a radius of the GRIN lens is a, and a refractive index of the GRIN lens at the radius a is na, the following is defined:g=NA/an0(2) wherein NA=(n02−na2)1/2  (2)
Here, reference character NA denotes a square root of a second power of a difference in the refractive indices of the GRIN lens at the core and the surrounding, which is termed a numerical aperture (hereinafter referred to as “NA”), which is an important parameter for representing the lens performance. The lens having a high NA is the lens having a high light-collecting ability, that is, the lens with an excellent lens property. A length (L) of the GRIN lens used as the collimator lens is set to a quarter length of the meander cycle of propagation light that propagates inside the GRIN lens, or set to its odd multiple. Provided that L1/4 is the quarter meandering wavelength, the following is defined;L¼=π/(2g)  (3)
wherein the length L of the GRIN lens is set as a quarter length of the meander cycle of propagation light shown in the equation (3) or set as its odd multiple, if a space between the opposing collimator lens is short. If the space becomes long, the length is typically set to be slightly longer than L1/4 to improve on a coupling loss. Hereinafter the case in which the space in between the collimator lens is short, will be explained.
FIG. 2 is a perspective view showing a single core collimator system having a pair of the GRIN lens 1a and 1b, which are opposingly arranged. The optical fibers 2a and 2b are connected to the GRIN lenses 1a and 1b at the end faces opposite to the facing planes by making the optical axes coincide. An optical signal transmission is carried out by outputting a light from one of the optical fiber 2a to the GRIN lens 1a connected thereto as a parallel light, collecting the parallel light at the other GRIN lens 1b, and introducing the light into the optical fiber 2b connected thereto. Consequently, in the collimator system using such GRIN lenses 1a and 1b, the optical axes of the opposing GRIN lenses 1a and 1b must precisely coincide, as well as the optical axes of the GRIN lenses 1a and 1b and the optical axes of the optical fibers 2a and 2b, to reduce the coupling loss.
FIG. 3 is a schematic perspective and explanatory view showing the fiber optic collimator array system in which a pair of the optical fiber collimator arrays 1 are opposingly arranged. In between both the fiber optic collimator arrays 1, two-staged MEMS-type (micro electromechanical systems) optical switch arrays 3 having 2 degrees of freedom are inserted. At end faces located opposite of the facing plane of each of the GRIN lenses for each of the fiber optic collimator arrays 1, the optical fibers 2a and 2b are connected by making the optical axes coincide so as to construct the optical fiber arrays 2 which are a group of these optical fibers. An optical signal transmission is carried out by outputting a light from each optical fiber 2a at one end, which is converted to a parallel light at the GRIN lens 1a connected thereto, reflecting two times at the optical switches 3a and 3b of the optical switch arrays 3, changing a direction of the parallel light by changing a reflection angle, collecting the parallel light at each GRIN lens 1b at the other end, and introducing it into the optical fiber 2b connected thereto. An optical path of the light is switched by appropriately changing the reflection angles of micro-mirrors 3c having 2 degrees of freedom comprised of two-staged MEMS-type optical switch arrays 3. Consequently, in such a collimator system, the optical axes of the opposing GRIN lenses 1a and 1b must precisely coincide respectively, as well as the optical axes of each of the GRIN lenses 1a and each of the optical fibers 2a, to reduce the coupling loss.
Here, FIG. 4 shows an actual example of a single core collimator system loading the optical isolator element. The industry-wide standard for an optical fiber core wire 7 is 0.9 mm. This optical fiber core wire 7 with built-in GRIN lens 1a and optical fiber 2a of 1.8 mm is inserted, adhered, and fixed to both ends of the optical fiber lens holder (metal) 8 of a concentric circle which is controlled to an outer dimension of 3 mm and inner diameters of 1.8 mm and 0.9 mm. The core wire 7 is made by covering the optical fibers 2a and 2b with a plastic and the like. Because the precision in the optical axis alignment of the optical fiber 2a and the GRIN lens 1a is influenced by the processing precision of the inner diameters of the optical fiber lens holder 8 of the concentric circle, a high level of processing precision is demanded. The optical fiber lens holder 8 is inserted into a collimator holder (metal) 9 controlled to the outer dimension of 10 mm and the inner diameter of 3 mm, and fixed to the collimator holder 9 by a fixing flange 11. Because the optical axis alignment of the single core collimator system facing each other accordingly is influenced by the processing precision of the inner diameter of the collimator holder 9 and the outer dimension and inner diameters of the optical fiber lens holder 8, a high level of processing precision is demanded. In the single core collimator systems which are opposingly arranged like this, the light that is output from the optical fiber 2a is collected at the GRIN lens 1a, which is output as approximately a parallel light 6. The substantially parallel light 6 is collected at the GRIN lens 1b at another end, and input to the optical fiber 2b. The collimator system functions accordingly. Various optical elements 10, such as an optical isolator component of the present example, are included and fixed between the opposing pair of collimator systems.
However, there are typically problems that occur such as processing precision of the devices and alignment precision from the point of view of the production technology (in the example of FIG. 4, processing precision of various holders 8 and 9, and tolerance precision of the optical fiber core wire and the GRIN lens 1a), including axial displacements in various directions when the GRIN lenses 1a and 1b are disposed so as to face each other as shown in FIG. 4. As shown in FIG. 2, the ideal optical axis is expressed by a letter C which is common to the GRIN lenses 1a and 1b, and the optical fibers 2a and 2b. When the Z direction is defined as a direction parallel to the optical axis C, the X direction is defined as a direction perpendicular to the horizontal direction, and the Y direction is defined as a direction perpendicular to the vertical direction, as the possible axis displacements between the facing lens, there occurs a displacement in the X direction, an inclination angle in the X direction θx, a displacement in the Y direction, and an inclination angle in the Y direction θy.
Moreover, as shown in FIG. 3, in the case of the fiber optic collimator array system in which two-staged MEMS-type (micro electromechanical systems) optical switch arrays 3 having 2 degrees of freedom are inserted between the two fiber optic collimator arrays 1 and 1, an optical path of the light can be switched by appropriately changing the reflection angles of the micro-mirrors 3c having 2 degrees of freedom comprised of the two-staged MEMS-type optical switch arrays 3. However, the mirror angles of each of the micro-mirrors 3c inside the optical switch array are not all equal and will vary slightly. For this reason, even if the core between the fiber optic collimator arrays 1 is aligned perfectly, the mirror angle displacements inside the optical switch array may generate the optical axis displacements by that amount of variance. Thus, the MEMS-type optical fiber collimator array system typically generates a large axial displacement in comparison to the single core collimator system in which the pair of GRIN lens 1a and 1b, shown in FIG. 2, are ordinarily arranged so as to oppose each other.
Normally, a GRIN lens and an optical fiber are connected by using an adhesive. Adhesives such as that disclosed in U.S. Pat. No. 4,213,677 are used to fix the optical fiber and the GRIN lens. In this configuration, when a highly intensive light is input from an optical absorption of the adhesive, the optical characteristics deteriorate due to an alteration of the adhesive caused by the elevated temperature. In general, under the wavelength region used in optical communication, the optical adhesive has an absorption ranging from 1 to 5%, and an alteration temperature of about 400° C. at maximum. The adhesive fails to tolerate the light intensity of a several Watts class for the physical property of this range. Moreover, there are many factors that will deteriorate a yield of the product when the optical fiber and the GRIN lens are spliced by the adhesive, such as angle displacements, optical axis displacements, air bubbles caused by including air in the adhesive, and an increased light reflection at a connecting surface, thus it causes a problem of increased cost. Moreover, four optical axes including each one of the GRIN lenses and each one of the optical fibers must coincide precisely, and thus the implementation is expensive.
To solve the above-mentioned problems, as disclosed in U.S. Pat. Nos. 4,701,011 and 5,384,874, a structure adopting a GI (graded-index) optical fiber as the collimator lens has been proposed. The GI optical fiber is an optical fiber with a radially varying refractive index at the core. The GI optical fiber is made of quartz, which is the same as the material forming the optical fiber, which allows it to be fused and spliced to the optical fiber, and the tolerance against the highly intensive light is expected to be obtained. In this case, the normal GI optical fiber is formed by a chemical vapor deposition method. In the chemical vapor deposition, NA=0.38 is obtained (for example, refer to literature by P. B. O'Connor et al: Electron. Lett., 13 (1977) 170-171). However, when an amount of additives (such as GeO2, P2O5) is increased in order to obtain a higher NA than the above value, this GI optical fiber is practically poor in handling upon fabricating the collimator lens in terms of matching the thermal expansion property, such that the parent material is liable to crack by an increase of the thermal expansion coefficient, and in terms of controllability of the refractive index, such as being unable to obtain the higher NA.
Patent Document 1: U.S. Pat. No. 4,213,677
Patent Document 2: U.S. Pat. No. 4,701,011
Patent Document 3: U.S. Pat. No. 5,384,874